Radical Islamists likely from the group Boko Haram unleashed new attacks against Christians in Nigeria over the weekend.
Suicide bombers targeted three churches in the city of Kaduna during Sunday services, killing 21 people.
The Red Cross said the attacks triggered reprisal killings Monday, bringing the total death toll to 50.
Boko Haram has claimed responsibility for Sunday's series of attacks on Nigeria's Christians in recent months and is suspected to be behind the latest bombing.
This was the third Sunday in a row that deadly attacks have been carried out against a Nigerian congregation.
Christian leaders encouraged believers not to let the attacks keep them from attending services.